Method and apparatus for measuring refractive index and thickness of film

ABSTRACT

A method for measuring a refractive index and a thickness of a dielectric thin film formed on a substrate. The method comprises the following four steps. A step for irradiating a thin film on a substrate with a monochromatic light of a wavelength λ, changing the incident angle thereof so as to measure a change of the energy reflection ratio or reflectance in response to changes of the incident angle and detect two incident angles θ1 and θ2 which correspond to two extreme values of the reflectance change. A step for irradiating the thin film with a monochromatic light of a wavelength λ&#39;, changing an incident angle thereof so as to measure change of reflectance in response to the change of the incident angle and detect an incident angle θ3 which corresponds to an extreme value of the energy change. A step for calculating the refractive indices and thicknesses of the thin film, on the basis of the incident angle values θ1 and θ2, changing the interference degree number used as a parameter. And a step for determining the thickness of the thin film and the refractive indices thereof with respect to the light of a wavelength λ and the light of a wavelength λ&#39;, on the basis of the incident angle value θ3 and the data calculated as mentioned above. The method makes it possible to easily and accurately carry out the measurement.

DESCRIPTION

1. Field of the Invention

The present invention relates to a method and an apparatus for measuringthe refractive index and thickness of a dielectric thin film formed on asubstrate.

2. Background Art

In connection with the semiconductor related techniques or the like, ithas become very important to develop an art for non-destructivelymeasuring the refractive index and thickness of a dielectric thin filmformed on a substrate.

In the state of art, a method called "LASER VAMFO" (LASER USING VARIABLEANGLE MONOCHROMATIC FRINGE OBSERVATION) is known as method for measuringthe refractive index and thickness of a thin film aiming to heighten theaccuracy of the measurement (see REFRACTIVE INDEX DISPERSION INSEMICONDUCTOR RELATED THIN FILMS: IBM J. RES. DEVELOP. MAY 1973).However, this known method has problems in that a large scale apparatusis necessitated for carrying out the method since a monochrometer mustbe used for the measurement and that it takes a relatively long time forthe measurement.

SUMMARY OF THE INVENTION

The present invention was made considering the above mentioned problemsof the state of the art. Therefore, it is an object of the presentinvention to provide a novel method and apparatus for measuring therefractive index and thickness of a dielectric thin film formed on asubstrate wherein it becomes possible to carry out the measurementeasily and accurately.

In accordance with the present invention, there is provided a method formeasuring the refractive index of a dielectric thin film formed on asubstrate, the method comprising the following four steps from a firststep to a fourth step.

The first step is to irradiate a thin film on a substrate with amonochromatic light having a wavelength λ, changing an incident anglethereof so as to measure change of energy reflection ratio in responseto the change of the incident angle and detect two incident angles θ1and θ2 which correspond to an arbitrary two of the extreme values of theenergy reflection ratio change, respectively. The second step is toirradiate the thin film on the sutstrate with a monochromatic lighthaving a wavelength λ', changing an incident angle thereof so as tomeasure change of energy reflection ratio, i.e., reflectance in responseto the change of the incident angle and detect one incident angle θ3which corresponds to on arbitrary one of the extreme value of the energychange. The third step is to calculate the refractive indices andthicknesses of the thin film, on the basis of the incident angle values-θ1 and θ2, with respect to a series of the interference degree numbersbeing used as parameters of the calculation. The fourth step is todetermine the thickness of the thin film and the refractive indexthereof with respect to the light having a wavelength λ and the lighthaving a wavelength λ', respectively, on the basis of the incident anglevalue θ3 and the values of the refractive indices and thicknessescalculated in the third step. The sequence of the steps may be in theorder of the above mentioned steps from the first step to the fourthstep. Or otherwise, the order of the second step and the third step maybe exchanged for each other.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory view for explaining an embodiment of thepresent invention;

FIG. 2 is an explanatory view for explaining another embodiment of thepresent invention; and

FIG. 3 is an explanatory view for explaining the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principle of the present invention will now be explained hereinafterwith respect to an actual example of the invention. FIG. 3 represents adielectric thin film 0 coated on a substrate 1 having a refractive indexn_(s) in a state in which a light irradiates the thin film at anincident angle θ and the light is reflected by the thin film.

Objects which are to be measured are refractive index n_(f) andthickness d of the thin film 0.

When the incident angle θ changes, the energy reflection ratio orreflectance also changes accordingly. The curve of the change of theenergy reflection ratio has a convex peak and a concave bottom due to aninterference phenomenon.

On the condition that the refractive index n_(s) is larger than therefractive index n_(f) (n_(s) >n_(f)), the energy reflection ratio ismaximized at an incident angle θ which satisfies the following equation.##EQU1##

Whereas the energy reflection ratio or reflectance is minimized at anincident angle θ which satisfies the following equation. ##EQU2##

On the other hand, on the condition that the refractive index n_(s) issmaller than the refractive index n_(f) (n_(s) <n_(f)), the energyreflection ratio or reflectance is minimized when the equation (1) issatisfied, whereas the energy reflection ratio or reflectance ismaximized when the equation (2) is satisfied.

When the refractive index n_(s) is larger than the refractive indexn_(f) (n_(s) >n_(f)) and when two minimum extreme values of the energyreflection ratio are obtained at incident angles θ1 and θ2,respectively, the following equations are derived from the equation (2)wherein the interference degree number m₁ and m₂ are represented by awhole number plus a half of one. ##EQU3##

By eliminating the value d of the film thickness from the above twoequations, the following equation with regard to the refraction indexn_(f) is obtained.

    n.sub.f ={(m.sub.1.sup.2 ·SIN.sup.2 θ2-m.sub.2.sup.2 ·SIN.sup.2 θ1)/(m.sub.1.sup.2 -m.sub.2.sup.2)}1/2(4)

An actual example is described below.

The substrate 1 of FIG. 3 is made from Si and the thin film 0 is madefrom SiO_(z). A laser beam of He--Ne having a wavelength 6328 Å isirradiated onto the thin film, changing the angle of incidence. Theenergy reflection ratio is detected or reflectance in relation to theangle of incidence. The two incident angle values θ1 and θ2 whichminimize the energy reflection ratio are 36.4 degrees for θ1 and 60.2degrees for θ2, respectively. In this case, relation between theinterference degree numbers m₁ and m₂ can be represented as m₁ =m₂ +1,since the minimum extreme values at the incident angles θ1 and θ2 areadjacent to each other representing at the angle 36.4 degrees first andat the angle 60.2 degrees subsequently.

The equation (4) mentioned above is calculated by substituting the abovementioned values for θ1 and θ2 in the equation (4). The calculationresult is substituted for the corresponding factor in the abovementioned equations (3-1) and (3-2) to calculated the equations so thatthe refractive index n_(f) and the film thickness d can be obtained foreach number of a series of the interference degree number m₁ which isused as a parameter of the calculation. A part of the calculation resultis represented in a table 1 below.

                  TABLE 1                                                         ______________________________________                                        m.sub.1        n.sub.f d (Å)                                              ______________________________________                                        5.5            1.25091 15802.8                                                6.5            1.32799 17311.1                                                7.5            1.401   18698.1                                                8.5            1.47049 19989.1                                                9.5            1.5369  21201.7                                                10.5           1.60061 22348.5                                                ______________________________________                                    

With regard to the thin film of SiO_(z), a genuine value of therefractive index is 1.470 and a genuine value of the film thickness is20000 Å. Therefore, a genuine value of the interference degree numberfor the incident angle θ1 is 8.5. However, the genuine value 8.5 can notbe selected from the table in accordance with the process so farmentioned above. Accordingly, it is not able to determine the genuinevalues of the refractive index and the thickness of the thin film fromthe table.

Referring again to the measuring process so far mentioned above, thestep in which the incident angles θ1 and θ2 are detected is the firststep referred to in the beginning portion of this disclosure of theinvention. Also, the third step mentioned there is the step in which therefractive index and the thickness of the thin film are calculated foreach of the interference degree numbers used as parameters of thecalculation on the basis of the incident angles θ1 and θ2 detected inthe first step.

After that, a laser beam of He--Ne having a wavelength 5941 Å which isdifferent from the wavelength 6328 Å used in the first step mentionedabove is irradiated to the thin film so as to detect an incident angleθ3 which minimizes the energy reflection ratio or reflectance withrespect to the He--Ne laser beam having the wavelength 5941 Å. Thedetection result of the incident angle θ3 was 24.5 degrees. The abovementioned two wavelengths are close to each other and the incident angleθ3 is close to the angle θ1. Therefore, the interference degree numberof the laser beam irradiated with the incident angle θ3 is supposed tobe the same as the interference degree number m₁ of the laser beamirradiated with the incident angle θ1 or slightly larger than m₁ by 1 or2.

By solving the equation (1) with respect to the refractive index n_(f),the following equation

    n.sub.f ={(m.sup.2 ·λ.sup.2 /4d.sup.2)+SIN.sup.2 θ}1/2(5)

can be obtained. In this equation, the film thickness d is constant withrespect to the wavelength. Therefore, the following calculation can becarried out. The incident angle θ3=24.5 is substituted for the θrepresented in the right side of the equation. Also, with respect to thefilm thickness d, each of the series of values of the film thicknesscalculated in the third step mentioned above is substituted for the filmthickness d represented in the right side of the equation. Each of thefilm thickness values calculated in the third step is derived bychanging the interference degree number m₁ as the parameter of thecalculation. With respect to the interference degree number m in theequation, m is assumed to be m₁, m₁ +1, and m₁ +2, respectively, so thatthe refractive index of the thin film is calculated from the equation(5) with regard to each of the interference degree numbers m₁, m₁ +1,and m₁ +2 in which m₁ is changed as the parameter of the calculation. Apart of the calculation result is represented in tables 2 to 4. In thetable 2, m₁ is substituted for the interference degree number mrepresented in the right side of the equation (5). Also, in tables 3 and4, m₁ +1 and m₁ +2 are substituted for the interference degree number mrepresented in the equation, respectively.

                  TABLE 2                                                         ______________________________________                                        m.sub.1   n.sub.f d           m    n.sub.f '                                  ______________________________________                                        5.5       1.25091 15802.8     5.5  1.11392                                    6.5       1.32799 17311.1     6.5  1.18997                                    7.5       1.401   18698.1     7.5  1.2616                                     8.5       1.47049 19989.1     8.5  1.32948                                    9.5       1.5369  21201.7     9.5  1.39412                                    10.5      1.60061 22348.5     10.5 1.45594                                    ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        m.sub.1   n.sub.f d           m    n.sub.f '                                  ______________________________________                                        5.5       1.25091 15802.8     6.5  1.29028                                    6.5       1.32799 17311.1     7.5  1.35213                                    7.5       1.401   18698.1     8.5  1.4126                                     8.5       1.47049 19989.1     9.5  1.4714                                     9.5       1.5369  21201.7     10.5 1.52845                                    10.5      1.60061 22348.5     11.5 1.5838                                     ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        m.sub.1   n.sub.f d           m    n.sub.f '                                  ______________________________________                                        5.5       1.25091 15802.8      7.5 1.46952                                    6.5       1.32799 17311.1      8.5 1.51637                                    7.5       1.401   18698.1      9.5 1.56517                                    8.5       1.47049 19989.1     10.5 1.61453                                    9.5       1.5369  21201.7     11.5 1.66374                                    10.5      1.60061 22348.5     12.5 1.71243                                    ______________________________________                                    

In the above tables 2 and 3, the refractive index n_(f) represents thecalculation result with regard to the wavelength 6328 Å which is usedfor the calculation in the third step, whereas the refractive indexn_(f) ' listed in the right end column of the tables represents thecalculation result with regard to the wavelength 5941 Å which is usedfor the calculation in accordance with the equation (5). Reviewing therefractive indices n_(f) and n_(f) ' in comparison to each other in thetables 2 to 4, it can be seen that, in the table 2, n_(f) is alwayslarger than n_(f) ' (n_(f) >n_(f) ') regardless of the value of theparameter m₁. Whereas, in the table 4, n_(f) is always smaller thann_(f) ' (n_(f) <n_(f) ') regardless of the value of the parameter m₁.Also, the difference between the refractive indices n_(f) and n.sub. fis larger than the refractive index change due to dispersion. Therefore,it is not adequate to adopt m₁ or m₁ +2 as the interference degreenumber m. On the other hand, in the table 3, with respect to therefractive indices n_(f) and n_(f) ', the larger refractive index ischanged between the two according to the value of the parameter m₁. Thedifference between the two refractive indices is minimized when m₁ =8.5wherein the difference n_(f) '-n_(f) =0.00091. Consequently, from thefact mentioned above, it is able to specify the thickness of the thinfilm to be 19989.1 Å. Also, the refractive indices for the wavelengths6328 Å and 5941 Å can be specified as 1.47049 and 1.4714, respectively.The refractive index for wavelength 5941 Å is slightly larger than therefractive index for wavelength 6328 Å, which is in accord withSellmeier's Law of Dispersion according to which the refractive index ofa dielectric material with regard to light in a visible range isslightly decreased as the wavelength of the light increases.

The fourth step mentioned before comprises the above mentioned processin which the refractive index and the thickness of the thin film arespecified on the basis of the incident angle θ3 obtained in the secondstep and the data obtained in the third step.

As can be seen from the above description, the fourth step includes thecalculation to obtain the refractive index n_(f) ' in accordance withthe equation (5) on the basis of the data calculated in the third step,i.e., the data of film thickness with respect to the interference degreenumber m₁ used as a parameter of the calculation and the incident angleθ3 detected in the second step and it also includes the calculation tospecify the film thickness and the refractive index for each ofdifferent wavelengths by comparing the refractive index n_(f) ',calculated as mentioned above, with the refractive index n_(f),calculated in the third step, so as to choose a parameter m₁ which makesthe two refractive indices closest to each other and makes therefractive index with respect to the shorter wavelength longer than withrespect to the longer wavelength.

In the calculation to obtain the refractive index n_(f) ', thecalculation is carried out in such a way that the interference degreenumber m in the equation (5) is increased one by one from the parameterm₁ or decreased one by one from the parameter m₁ so that the calculationis repeated until the larger refractive index is changed between the tworefractive indices n_(f) and n_(f) ' in response to the change of theparameter m₁. If the wavelengths of the two lights are too far differentfrom each other, the difference between the degree numbers of theextreme values which correspond to the wavelength of the light maybecome large as well as the dispersion of the refractive index withrespect to the wavelength. Therefore, it is desirable to select the twowavelengths of the light which differ not so much from each other tominimize the difference therebetween.

The present invention is further described below referring to anembodiment in actual use.

A sample Ob to be measured comprises a substrate of Si on which a thinfilm of SiO_(z) is coated by a sputtering method. The measuring sampleOb is placed on a turn table 10 as illustrated in FIG. 1. An arm 12 isarranged coaxial with the turn table 10. The arm 12 is arranged in sucha way that when the turn table 10 is rotated by a driving system 28, thearm 12 is rotated at a rotational speed twice that of the turn table 10so that the rotational angle of the arm 12 is twice as that of the turntable 10. A sensor 14 for detecting the light reflected from the sampleOb is mounted on the arm 12 at an end thereof.

Numeral 16 designates a laser source of He--Ne having a wavelength of6328 Å and numeral 18 designates a laser source of He--Ne having awavelength of 5941 Å. Numerals 20 and 22 designate shutters. Numeral 24designates a dichroic mirror. The dichroic mirror 24 is designed in sucha way that the transmission factor thereof with regard to the laser beamof wavelength 6328 Å is large whereas the reflection ratio thereof withregard to the laser beam of wavelength 5941 Å is large. Therefore, itbecomes possible to change the wavelength of the incident lightirradiated to the measuring sample Ob by driving the shutters 20 and 22.

The incident light is adjusted to become an S-polarized light withrespect to the incident plane by a Glan-Thomson prism 26. The reason forusing the S-polarized light as the incident light is that if aP-polarized light is used, the difference between the maximum extremevalue and the minimum extreme value of the energy reflection ratiobecomes unclear due to the Brewster's angle.

The incident angle is set to be zero at the starting point of themeasurement and increased in the measuring process by rotating the turntable 10 and the arm 12. In this case, the sensor 14 always receives thereflection light from the sample without fail since the rotation angleof the arm 12 is arranged to be twice as that of the turn table 10.

The turn table 10 and the arm 12 are driven to be rotated by the drivingsystem 28 which is controlled by a controlling system 30. Thecontrolling system 30 also controls the laser sources 16 and 18, theshutters 20 and 22, a data processing system 34 and recording system 32.The data processing system 34 carries out the calculation of the thirdstep and the fourth step as mentioned above. The recording system 32records the result of the above mentioned calculation. The controllingsystem 30 and the data processing system 34 can be realized by acomputer system. Also, the recording system 32 may comprises a displaymeans such as a CRT (cathode ray tube) or a printer means.

First in operation, the shutter 22 is closed and the other shutter 20 isopened so that the laser beam of wavelength 6328 Å is adjusted to be theS-polarized light and irradiated onto the sample Ob placed on the turntable 10 which is rotated to change the incident angle of the light withrespect to the sample. The energy reflection ratio or reflectance of thelight reflected from the sample changes in response to the change of theincident angle. The change of the energy of reflection is detected bythe sensor 14 as the output change thereof so that two incident angleswith which the energy reflection ratio or reflectance becomes a maximumextreme value are detected. The two incident angles θ1 and θ2 aredetected as 28.5 degrees and 48.8 degrees at which angles the energyreflection ratio curve represents two peak extreme values which areadjacent to each other (the first step). Therefore, the interferencedegree numbers of the two incident angles are different from each otherby a degree number of 1.

After that, the shutter 20 is closed and the shutter 22 is openedinstead so that the incident light is changed to the laser beam ofwavelength 5941 Å. With the use of this laser beam, one incident angleθ3 with which angle the energy reflection ratio or reflectance becomes amaximum extreme value is detected. The detected incident angle θ3 is42.7 degrees (the second step). With regard to this measuring sample,the refractive index of the thin film is supposed to be lower than thatof the substrate. Therefore, the interference degree numbers m₁ and m₂represented in the right sides of the equations (3-1) and (3-2) arenatural numbers.

In accordance with the equations (3-1), (3-2) and (5), the refractiveindex n_(f) and the film thickness d are calculated using theinterference degree number m₁ as a parameter of the calculation (thethird step).

After that, the data calculated as mentioned above and the incidentangle 42.7 degrees are substituted for the factors in the equation (5)so that the refractive index n_(f) ' for the wavelength 5941 Å iscalculated using m₁ +1 as the interference degree number m. Also, aparameter m₁ is selected which parameter makes a state in which thedifference between the refractive indices n_(f) and n_(f) ' becomessmallest and in which n_(f) is smaller than n_(f) ' (n_(f) <n_(f) ').

The refractive index and the film thickness corresponding to thisparameter are specified as a genuine value of measurement result (thefourth step). A part of the calculation result is represented in table5. Actually, the calculation was repeated changing the parameter m₁ from2 to 20.

                  TABLE 5                                                         ______________________________________                                        m.sub.1  n.sub.f d            m   n.sub.f '                                   ______________________________________                                         8       1.29295 21063.7       9  1.31633                                      9       1.35657 22424.1      10  1.3716                                      10       1.41739 23706.5      11  1.42478                                     11       1.47573 24923        12  1.47607                                     12       1.53188 26082.8      13  1.52566                                     13       1.58606 27193.2      14  1.5737                                      ______________________________________                                    

As can be seen from the table 5, the refractive index which is specifiedas the genuine value of the measurement is the one which minimizes thedifference between n_(f) and n_(f) ', i.e., the one which is selectedwhen the parameter m₁ is 11 in which the n_(f) and n_(f) ' are1.47573(6328 Å) and 1.47607(5941 Å), respectively. Also, the filmthickness is 24923 Å.

As mentioned above with reference to an embodiment of the presentinvention, it is possible to obtain a genuine value of the refractiveindex and the film thickness by a calculation on the basis of incidentangles which make a state in which a peak extreme value of the energyreflection ratio or reflectance is represented.

Another embodiment of the present invention is illustrated in FIG. 2.

A measurement sample Ob comprises a glass substrate of Pyrex (tradename) on which a nitride film is coated by a plasma CVD method. Objectsof the measurement are the refractive index and the thickness of thenitride film.

The incident light irradiated onto the sample is arranged in such a waythat the diameter of the laser beam from the light source is expanded bya beam expander 36, then the beam is converged by a condenser lens 38 toirradiate the sample Ob which is unmovably supported. With thisarrangement, it becomes possible to irradiate the sample by a light fromcontinuously different angles of incidence at a time.

Also, the reflection light reflected from the sample is collectivelyreceived by a photosensor array 40 at a time. The output of thephotosensor array 40 is read in a time sequence by a data processingsystem by driving a controlling system similar to the one shown in FIG.1 so that the incident angles which give an extreme value are detectedso as to carry out the calculation on the basis of the detected data.The data processing system and the recording system are similar to thoseof the embodiment of FIG. 1.

In the state that the shutter 20 is opened to irradiate the sample withthe incident light of wavelength 6328 Å, the incident angles θ1 and θ2which make the energy reflection ratio or reflectance minimized aredetected (the first step). The detection data were 31.4 degrees and 56.5degrees. Also, in the state that the shutter 22 is opened to irradiatethe sample with the incident light of wavelength 5941 Å, the incidentangle θ3 which makes the energy reflection ratio minimized is detected(the second step). The detection data was 56.8 degrees.

In this case, the refractive index n_(s) of the substrate is smallerthan the refractive index n_(f) of the thin film. Therefore, theinterference degree numbers m₁ and m₂ of the right sides of theequations (3-1) and (3-2) are a natural number. The third step and thefourth step are carried out in a same manner as in the embodimentmentioned before. A part of the calculation result is represented intable 6. Actually, the calculation was carried out changing theparameter m₁ from 2 to 20.

                  TABLE 6                                                         ______________________________________                                        m.sub.1  n.sub.f d            m   n.sub.f '                                   ______________________________________                                        12       1.71042 23305.6      13  1.74205                                     13       1.77119 24297.8      14  1.79478                                     14       1.82996 25251        15  1.84602                                     15       1.88692 26169.5      16  1.87588                                     16       1.94221 27056.9      17  1.94448                                     17       1.99598 27916        18  1.99189                                     ______________________________________                                    

As can be seen from table 6, the refractive indices specified as thegenuine values of the measurement are 1.94221(6328 Å) and 1.94448(5941Å) when the parameter m₁ is 16. Also, the film thickness is specified tobe 27056. 9 Å as the genuine value of the measurement.

As mentioned above, in accordance with the present invention, a novelmethod for measuring the refractive index and thickness of a thin filmcan be provided. The method of the present invention comprising theabove mentioned arrangement makes it possible to easily and accuratelymeasure the refractive index and the thickness of a dielectric thinfilm.

Note that the above mentioned explanation refers to the embodiments inwhich an absorption coefficient of the substrate is zero or negligible.When the absorption coefficient of the substrate is large, for instancewhen a substrate of aluminum is used, it is necessary to correct thecalculated value of the film thickness d obtained in the third step andthe fourth step mentioned above. Such correction can be made inaccordance with a known method of "PHASE-SHIFT THICKNESS CORRECTION"which is disclosed in a document "PHASE-SHIFT CORRECTION IN DETERMININGTHE THICKNESSES OF TRANSPARENT FILMS ON REFLECTIVE SUBSTRATES (SOLIDSTATE ELECTRONICS PERGAMON PRESS 1968 VOL. 11, PP957-963). Subsequentcalculation process after the correction is made is the same as theprocess of the above mentioned embodiments.

INDUSTRIAL APPLICABILITY

The present invention can be applied to a device for forming thin filmssuch as dielectric films, wiring pattern films and semiconductor filmsin a process for manufacturing semiconductor related devices. Forexample, the present invention can be applied to a vacuum evaporationdevice, a sputtering device, a PVD device, a CVD device and an epitaxialdiffusing device.

I claim:
 1. An apparatus for measuring a refractive index and athickness of a thin film formed on a substrate, comprising:a first lightsource for emitting a first monochromatic light having a wavelength λ; asecond light source for emitting a second monochromatic light having awavelength λ', which is different from that of said first monochromaticlight; a light changing means for changing a light to be irradiated ontosaid thin film by selecting one of said emitted first monochromaticlight and said emitted second monochromatic light; an incident anglechanging means for changing an incident angle of said selectedmonochromatic light and for detecting said changed incident angle ofsaid selected monochromatic light; a detection means adapted to detect areflection light reflected from said substrate, to measure a reflectanceof said detected reflection light, to determine two arbitrary minima ortwo arbitrary maxima of said measured reflectance corresponding to achange of said incident angle of said first monochromatic light on thebasis of said detected incident angle of said emitted firstmonochromatic light to thereby determine respectively two incidentangles θ₁ and θ₂ corresponding to said determined two arbitrary minimaor said determined two arbitrary maxima when said emitted firstmonochromatic light is selected by said light changing means, and todetermine a single minimum or maximum of said measured reflectancecorresponding to a change of said incident angle of said secondmonochromatic light on the basis of said detected incident angle of saidemitted second monochromatic light to thereby determine an incidentangle θ₃ corresponding to said single minimum or maximum when saidemitted second monochromatic light is selected by said light changingmeans; and a calculation means adapted to calculate refractive indicesand thicknesses of said thin film on the basis of said wavelength λ andsaid determined two incident angles θ₁ and θ₂ as varying orders ofinterference m₁ and m₂ as parameters according to the followingequations: ##EQU4##

    n.sub.f ={(m.sub.1.sup.2 sin .sup.2 θ.sub.2 -m.sub.2.sup.2 sin .sup.2 θ.sub.1)/(m.sub.1.sup.2 -m.sub.2.sup.2)}1/2

wherein m₁ and m₂ represent said orders of interference, n_(f)represents said refractive index of said thin film, and d representssaid thickness of said thin film, to calculate refractive indices n'_(f)of said thin film on the basis of said wavelength λ', said determinedincident angle θ₃ and said calculated thickness d as varying an order ofinterference m as a parameter according to the following equation:

    n'.sub.f ={(m.sup.2 ·λ'.sup.2 /4d.sup.2)+sin .sup.2 θ.sub.3 }1/2

to compare a value of each of said calculated refractive indices n_(f)with another value of each of said calculated refractive indices n'_(f)with respect to a pair of said calculated refractive indices n_(f) andn'_(f) which represent an identical thickness, to determine a pair ofrefractive indices n_(f) and n'_(f) as genuine refractive indices forsaid wavelengths λ and λ', and to specify a film thickness correspondingto said specified refractive indices n_(f) and n'_(f) as a genuine valueof said thickness of said thin film.
 2. An apparatus for measuring arefractive index and a thickness of a thin film according to claim 1,wherein said first light source comprises a first laser source emittinga first laser beam, and said second light source comprises a secondlaser beam having a wavelength different from a wavelength of said firstlaser beam.
 3. An apparatus for measuring a refractive index and athickness of a thin film according to claim 1, wherein said lightchanging means comprises a shutter disposed on optical paths of each ofsaid emitted first monochromatic light and said emitted secondmonochromatic light.
 4. An apparatus for measuring a refractive indexand a thickness of a thin film according to claim 1, wherein saidincident angle changing means comprises a rotational turn table on whichsaid substrate is mounted.
 5. An apparatus for measuring a refractiveindex and a thickness of a thin film according to claim 4, wherein saidincident angle changing means comprises a sensor disposed on an armwhich is adapted so as to be coaxial with said turn table and adapted sothat a rotation angle of said arm is always twice as large as that ofsaid turn table.
 6. An apparatus for measuring a refractive index and athickness of a thin film according to claim 1, wherein said lightchanging means comprises an optical system adapted so that said selectedmonochromatic light is first expanded in a diameter of a beam thereofand then said expanded monochromatic light is converged on saidsubstrate by a condenser lens in such a manner that monochromatic lightshaving wavelengths different from one another are simultaneouslyirradiated on said thin film.
 7. An apparatus for measuring a refractiveindex and a thickness of a thin film according to claim 6, wherein saiddetection means comprises a photosensor array capable of detectingsimultaneously reflection lights reflected from said substrate atdifferent reflection angles.
 8. An apparatus for measuring a refractiveindex and a thickness of a thin film according to any one of claim 1 to7, further comprising a polarization means for changing said selectedmonochromatic light to an s-polarized light.